The present invention is concerned with enhancing the appearance of display matrixes in which each pixel comprises an LED lamp.
A problem in designing LED lamp matrixes is that of achieving uniformity so that all the lamps give the same light output. The light output of a new LED at a given temperature is dependent on its light efficiency, measured as light intensity at unit current, and on the operating current. Also LEDs are subject to intensity degradation, i.e. fading, with prolonged use.
For most types of LED lamp the light efficiency, often expressed in the form of luminous intensity at 20 mA, can vary from sample to sample by about 5:1. For some types, the diodes are sorted from the production line to have a lower ratio of maximum to minimum light efficiency form sample to sample, for example 2:1.
In an LED matrix with multiplexed drive, current is limited in each LED, usually by means of a resistor that is in series with the LED when it is turned on, and the matrix is preferably driven from a 5 volt supply to avoid reverse breakdown of the LEDs and to keep the power consumption low. The current, I, in a selected LED in such a case is given by: EQU I=(5-V.sub.L).div.R.sub.s
where V.sub.L is the forward voltage drop of the LED and R.sub.s is the value of the current limiting resistor. V.sub.L can vary from 1.8 to 3 volts for some types of LED, and using such types the current, I, can vary from a maximum value of 3.2/R.sub.s to a minimum value of 2/R.sub.s, i.e. in the ratio 3.2:2. Thus if the initial light efficiency varies by 2:1, the light output can vary by 3.2:1. Added to this are variations in intensity degradation with time, and variations due to the differences in the voltage drops across the switches routing the currents to the LEDs.
Yet another factor affecting uniformity of an LED display matrix is that the junctions of the LEDs are not all at the same temperature. Those that are on, or have recently been on, are hotter than those that have been off. The difference between the hottest and the coolest junction temperature at any one time can be as much as 50 degrees centigrade. Since the light intensity of an LED can drop by 1% per degree centigrade, this represents a further 2:1 mismatch in intensity. The effect is dynamic. The time constants of junction temperature change can be of the order of a second for the LED itself and tens of seconds for its heat sink, which is typically its printed circuit board.
Not only are there intensity mismatch effects, but there are also color mismatch effects. LED lamps can be initially mismatched in color, when received from the manufacturer, by as much as 11 nanometers in wavelength for some green LEDs. Furthermore, LEDs are subject to dynamic color mismatch, due to dynamic temperature mismatch of the lamps. Further still, LEDs are subject to color degradation, i.e. change of color with prolonged use, which can itself cause color mismatch, since the lamps are not used equally and, in any case, are not guaranteed to have the same rate of degradation.